Researchers uncover processes behind the creation and evolution of natural aerosols, small atmospheric particles free from the influence of pollution, that can seed cloud formation.

From Carnegie Mellon University

A research group from the CERN Cloud experiment, including scientists from Carnegie Mellon University’s College of Engineering and Mellon College of Science, have uncovered the processes behind the formation and evolution of small atmospheric particles free from the influence of pollution. Their findings are key to creating accurate models to understand and predict global climate change. The findings are published in the May 26 issue of Nature.

Clouds and aerosols-small airborne particles that can become the seeds upon which clouds form-are essential to climate predictions because they reflect sunlight back into space. Reflecting light away from Earth can have a cooling effect, masking some of the warming caused by greenhouse gases.

“The best estimate is that about one-third of the warming by greenhouse gas emissions is masked by this aerosol cooling, but the fraction could be as large as half and as little as almost nothing,” says Neil Donahue, professor of chemical engineering, engineering and public policy, and chemistry at Carnegie Mellon.

In order to have complete climate prediction models, scientists need to incorporate clouds and aerosols into their calculations, but understanding how new aerosol particles form and grow in the atmosphere, and how they affect clouds and climate, has been challenging.

Scientists involved with CERN’s CLOUD experiment study use a large chamber to simulate the atmosphere and track the formation and growth of aerosol particles and the clouds they seed. The latest research shows that new particles can form exclusively from the oxidation of molecules emitted by trees without the presence of sulfuric acid. Sulfuric acid largely arises from fossil fuels, so the new findings provide a mechanism by which nature produces particles without pollution.

“This softens the idea that there may be many more particles in the atmosphere today due to pollution than there were in 1750, and suggests that the pristine pre-industrial climate may have had whiter clouds than presently thought,” says Donahue.

The team’s research has lasting climate implications.

“Earth is already more than 0.8C than it was in the pre-industrial epoch, and this is with some masking by aerosol particles. As the pollution subsides, up to another 0.8C of hidden warming could emerge,” says Donahue.

Abstract

About half of present-day cloud condensation nuclei originate from atmospheric nucleation, frequently appearing as a burst of new particles near midday. Atmospheric observations show that the growth rate of new particles often accelerates when the diameter of the particles is between one and ten nanometres. In this critical size range, new particles are most likely to be lost by coagulation with pre-existing particles, thereby failing to form new cloud condensation nuclei that are typically 50 to 100 nanometres across. Sulfuric acid vapour is often involved in nucleation but is too scarce to explain most subsequent growth, leaving organic vapours as the most plausible alternative, at least in the planetary boundary layer. Although recent studies predict that low-volatility organic vapours contribute during initial growth, direct evidence has been lacking. The accelerating growth may result from increased photolytic production of condensable organic species in the afternoon, and the presence of a possible Kelvin (curvature) effect, which inhibits organic vapour condensation on the smallest particles (the nano-Köhler theory), has so far remained ambiguous. Here we present experiments performed in a large chamber under atmospheric conditions that investigate the role of organic vapours in the initial growth of nucleated organic particles in the absence of inorganic acids and bases such as sulfuric acid or ammonia and amines, respectively. Using data from the same set of experiments, it has been shown that organic vapours alone can drive nucleation. We focus on the growth of nucleated particles and find that the organic vapours that drive initial growth have extremely low volatilities (saturation concentration less than 10−4.5 micrograms per cubic metre). As the particles increase in size and the Kelvin barrier falls, subsequent growth is primarily due to more abundant organic vapours of slightly higher volatility (saturation concentrations of 10−4.5 to 10−0.5 micrograms per cubic metre). We present a particle growth model that quantitatively reproduces our measurements. Furthermore, we implement a parameterization of the first steps of growth in a global aerosol model and find that concentrations of atmospheric cloud concentration nuclei can change substantially in response, that is, by up to 50 per cent in comparison with previously assumed growth rate parameterizations.